PROJECT SUMMARY Interstrand crosslinks (ICLs) are the most toxic lesions in the cell and are known to be effective in cancer treatment. Understanding how ICLs are repaired in cancers will allow us to take advantage of these agents for cancer treatment, to avoid therapy resistance, and to develop biomarkers that can be used to prevent overtreatment for patients who are not responding to these agents. However, despite extensive studies in this area, it remains challenging to come up with a detailed hypothesis and reveal precisely how multiple DNA repair enzymes and pathways act together in ICL repair. This Program Project brings together experts in various aspects of DNA repair pathways and aims at addressing this difficult question. Project 4 focuses on a critical scaffold protein SLX4. Aim 1 of this project will determine the mechanisms underlying the cell cycle- dependent regulation of SLX4-containing protein complexes and investigate how SLX4 and MSH2 complexes act together to promote ICL repair. Aim 2 will define mechanistically how SLX4 and SLX4IP are recruited to DNA damage sites and participate in ICL repair. Aim 3 attempts to uncover the connection between SLX4 complex and FA pathway in ICL repair and further determine the significance of these repair components in cancer therapy. These studies, together with the ones proposed in Projects 1-3, will provide a framework of ICL repair in humans.